The field contains empty and occupied slots.
A slot can have one of the three states:
1) Available, where you can merge, drag or remove items.
2) Semi-available, where the item is visible, but you cannot move or remove it, only merge it. Once you merge the item in this slot, it becomes available.
3) Unavailable, where the slot is locked and the item is not visible.
It automatically becomes semi-available, if:
- you reach the required profile level;
- it is adjacent to an available slot (provided there is no profile level restriction).